Document handlers are devices that draw individual sheets from a stack of sheets, and sequentially allow the image on each sheet to be recorded, typically by a photoreceptor (in a “light-lens” or analog copier) or by a photosensitive device (in a digital copier, scanner, or facsimile). In a common arrangement, a copier has a conventional main platen, on which single sheets can be manually placed, as well as a smaller platen, typically adjacent the main platen, which is used by the document handler when sheets are being passed therethrough. In a typical design, when a single sheet is being recorded through the main platen, the photosensitive device with associated light source (or “scan head”) is moved relative to the platen to record the entire image; when the document handler is being used to expose images through the smaller platen, a photosensitive device is typically left stationary under the smaller platen, and the motion of the sheet caused by the document handler provides the necessary relative motion of each sheet past the photosensitive device.
For recording images on sheets that bear images on both sides thereof, there has typically been two general approaches. For scanners with a single scan head, the general approach is to use the scan head first to scan one side of the sheet, and then mechanically invert and re-feed the sheet so that the other side of the sheet is moved past the scan head. For scanners with two scan heads, each scan head is effectively placed on opposite sides of a paper path, so that both images on both sides of the sheet are recorded. A practical overall architecture for a two-scan-head input scanner can be seen in U.S. Published Patent Application 2004/0080791 A1, now U.S. Pat. No. 7,164,507.
In the practical use of any sophisticated scanning apparatus, it is occasionally desired to “calibrate” the imaging devices therein, i.e., record an image from a calibration surface of predetermined properties, such as reflectivity and color, and use the resulting calibration information in affecting data from subsequently-scanned images. Such calibration surfaces are typically formed on the apparatus itself, in a location where an imaging apparatus can record it, such as shown, for example, in U.S. Pat. Nos. 4,429,333 and 5,280,368.